Iron dissolved in various kinds of aqueous solutions causes many undesirable effects. Its removal has long been a problem in the oil and gas industry. As applied to workover and completion fluids used in hydrocarbon recovery, sometimes called clear completion brines, the background of the problem has been well described by Qu et al. in U.S. Pat. No. 7,144,512, as described below:                High density brines (completion brines) have been widely used in well completion and workover operations in oilfields in the past several decades. The completion brines are salt solutions typically having fluid densities ranging from about 8.4 ppg (pounds per gallon) to about 20 ppg. Depending on the density desired, a completion brine can be a one salt solution (e.g. NaCl, NaBr, CaCl2, CaBr2, ZnBr2 or formate salt in water), a two salt solution (e.g. CaCl2/CaBr2 or ZnBr2/CaBr2), or a three salt solution (e.g. ZnBr2/CaBr2/CaCl2). The composition of the brines determines the fluid properties such as pH, density, etc. Depending on the economics, a fluid can be used in a well and then purchased back to be cleaned and reused later.        At the conclusion of any completion or workover project, a substantial volume of “contaminated” or unneeded completion/workover fluid typically remains. Such fluids may be contaminated with any or all of the following: water, drilling mud, formation materials, rust, scale, pipe dope, and viscosifiers and bridging agents used for fluid-loss-control pills. Depending on their composition and level of contamination, these fluids may or may not have further practical or economic value. If it is deemed that the fluids have future use potential, they may be reclaimed. Conversely, if they are determined to have no further use, they must be disposed of in an environmentally responsible way.        The benefits derived from the use of solids-free fluids, and especially high-density brines, for completion and workover operations have been extensively documented in the literature. Unfortunately, the costs associated with the initial purchase and subsequent disposal of such brines has been a hindrance to their universal acceptance especially since the “use once and dispose” means of disposal is neither prudent nor economically sound.        Because of the relatively high cost and limited worldwide natural mineral resources available for producing medium- and high-density completion/workover fluids, it is essential that their used fluids be reclaimed. The reconditioned fluids must meet the same specifications as those of “new” or “clean” fluids. With respect to completion/workover fluids, the term “clean” denotes not only the absence of suspended solids but also the absence of undesirable colloidal or soluble species which are capable of undergoing adverse reactions with formation, formation fluids or other completion fluids to produce formation-damaging insoluble substances.        There are many known methods for removing contaminates from a brine solution. One approach is to remove suspended solids by filtration. Simple filtration processes, wherein the brine is filtered through a plate and frame type filter press with the use of a filter aid such as diatomaceous earth and then through a cartridge polishing filter, are effective to remove solid contamination but they have no effect on removing other types of contamination such as colloidal or soluble species. This is the case since colloidally dispersed and soluble contaminants cannot be removed by filtration without first treating the fluid to change the chemical and/or physical properties of the contaminants. The treatments required to salvage the fluid depend on the nature of the contaminants incorporated and their chemical and physical properties.        
It is widely recognized that iron is a common contaminant in completion fluids. Accumulation of iron in a brine completion fluid may cause formation damage and affect the productivity of a well. In addition, iron may cause other negative effects, such as cross-linking, gelling of polymers, and increased stabilization of crude/brine emulsions. Completion fluids tend to accumulate dissolved contaminants, such as iron, when deployed into the well. Completion fluids are often used more than once, e.g., recovered from a wellbore and then used again. Oilfield specifications, such as those set by the American Petroleum Institute (API), may set an upper limit for the iron concentration in completion fluids, which may be exceeded in brine recovered from the wellbore. Thus, the contaminated brine is generally treated to remove the iron (and other contaminants). As compared to other fluids used in the oil and gas industry (such as produced fluids, which may be used as frac fluids), high density completion fluids tend to have significantly higher levels of dissolved iron in the fluids due to the corrosive nature and pH of the salts such as zinc bromide. Further, depending on the formation pressure, different compositions/weights of brines are used as completion fluids, and in some cases, zinc-bromide containing brines are often used in such dense-fluid applications.
As the prior art recognizes, zinc containing high-density brines have proven to be the most difficult to treat for iron removal. Most of the zinc-containing brines have relatively low pH, which often leads to high iron contamination during use as “conventional” completion and/or workover fluids. Iron contamination in such fluids can reach several hundred or even thousand milligrams per liter. Further, iron in zinc brine solutions is more likely to be in a soluble and stable form. Because of the low solubility of oxygen in such solutions, a significant percentage of the iron contaminants exist as ferrous iron. As a result, precipitation of iron is challenging. For example, if the pH of a solution is raised in the attempt to precipitate out the dissolved iron, the useful zinc and calcium bromide salts typically present in high-density completion fluids will also precipitate. It is even more challenging to remove iron from a zinc bromide containing solution without also removing the zinc.
Some conventional methods for treating used brine to meet oilfield specifications rely on removal of iron therefrom using strong oxidizers, such as hydrogen peroxide, to oxidize and precipitate the iron out of solution. Other conventional methods rely on the dilution of the contaminated brines using less dense brines, such as calcium chloride brines. However, removing dissolved iron from a zinc-bromide brine using strong oxidizers also tends to remove the zinc from the solution, which lowers the density of the fluid. Additionally, the use of less expensive fluids to dilute contaminated brines not only reduces the concentration of iron, but also lowers the density of the fluid, which may make it less suitable or desirable than the heavier brines. Ou et al. (U.S. Pat. No. 7,144,512) provides a solution to remove iron from a well completion brine solution that comprises zinc bromide using an organic chelant to form a complexed metal precipitate. Vollmer et al. (U.S. Pub. No. 2014/0121138) provides a method that recovers zinc, nickel, and iron from spent brines and produced water using a hydrazine complex. Such a solution, however, removes the desired zinc salts from the completion fluid and is intended to reduce the cost of disposing the substantially zinc free fluid. Bae et al. (U.S. Pub. No. 2012/0145646) discloses a method for removing iron from an aqueous solution by changing the pH with the addition of acids and a phosphate salt that would, if present, remove zinc and other similar salts. Smith et al. (U.S. Publication No. 2009/0184056) discloses a technique that uses a cavitation device together with an oxidizing agent and the addition of lime. Isaac (U.S. Pat. No. 7,244,363) discloses a method that treats heavy halide brines with permanganates to remove iron and other heavy metal contaminants. Each of these references is incorporated herein by reference.
As mentioned above and as obvious to one of ordinary skill in the art, the existing techniques for high-density completion fluids suffer from many disadvantages. While dissolved iron is a well-known problem in the oil and gas industry, no one has developed a practical, inexpensive, and effective solution for iron removal from high-density completion fluids. A need exists for a novel way to remove iron while keeping zinc in solution in a high-density completions fluid.